The idea of the Golden Milestone, a reference point from which all distances are measured, is as old as Rome itself. The nominal datum point and centre of the Roman Empire was the Millarium Aureum - the original golden milestone, raised by Emperor Augustus in 20 BC (5) . This 2.5m tall and 1.2m diameter marble cylinder marked the point from which the major Imperial highways radiated - the Via Aurelia, Via Appia and Via Flaminia. The column was originally covered in gilt bronze, and on it the major cities of the empire and their distances are said to have been inscribed. This stone and even its exact location has been lost, as road construction in Rome in 1835 has erased the foundations, though it turned up two fragments of a large marble cylinder 1.2m diameter with traces of bronze facing and chiselled writing.
Succeeding centuries have not had the same clarity of vision, and the principle of a single datum for any city was lost for a long time. For many 17th to 19th century milestones the datum has often been to the nearest city wall, or to a variety of other points. There is even some indication that on the Milliarium Aureum the distances were to city gates in the Servian Wall rather than to the Golden Milestone itself!
It was the great expansion of road building in the 20th century that restored the idea of a single datum point for a city. That for London became a statue at Trafalgar Square, near Charing Cross - previously the distance to London had been cited from a number of points - the old London Stone of antiquity, Whitechapel for roads running east, Marble Arch to the west.
In America, the zero milestone is located outside the White House, erected in 1929.